Bridging NHCs

A special type of reaction is observed with the platinum(IV) complex [PtI(Me)3] which cleaves the Af,N,Af, A -tetraphenyltetraaminoethylene under reduction to form the dimeric cyclometallated mono(NHC) complex of platinum(II) iodide [Eq. (31)]. Cyclometallation with the same ligand is also observed for ruthe-nium. Additional cyclometallations with various substituents of NHCs have been reported for ruthenium(II), rhodium(III), iridium(I), palladium(II), " and platinum(II). In the case of iridium, alkyl groups can be activated twice. In rare cases like for nickel(II) /x-bridging NHCs have been obtained. ... 

A helical homobimetallic mercury(II) complex with a bridging bis(NHC) ligand serves as a starting point for a supramolecular assembly. Also tetrameric cyclic palladium(II) complexes have been obtained with bridging NHC-pyridine ligands. 

While the former was obtained directly by reaction of [Ybljfthf), ] with KlReCpj], the latter was synthesized via salt elimination from (Ybljfthf) ] and Na[RuCp(CO)2] followed by cracking of the isocarbonyl linkages in the primary polymeric product with 4-tert-butylpyridine (Scheme 3.3, top and middle) [13]. Using an iodo-bridged, NHC-stabilized dimer of neodymium and K[FeCp(CO)2], a complex with an unsupported Nd-Fe bond was obtained (Scheme 3.3, bottom) [14],... 

Figure 11.14 A trinudear Ag(l) duster with bridging NHC ligands.

Figure 6.3 Indenyl- and fluorenyl-bridged NHC early transition and rare earth metal complexes (Dipp = 2,6-diisopropylphenyl).

These results suggest that imidazolidin- and imidazol-based skeletons transfer similar amounts of electron density to the metal. The conclusion that changes in the bridge of the NHC skeleton have such a small effect on the electronic properties of the NHC is quite surprising, considering that SIMes- and IMes-based catalysts often show remarkably different catalytic behaviour. It is still unclear if these small changes in the electronic properties of the NHC ligand confer such different catalytic behaviours, or other effects (steric, flexibility, etc.) should be invoked. 

Rh(III)(NHC) hydrides have been studied as catalysts for this type of hydrogenation. The products from the reaction of Rh(I) complexes with are dependent on the natnre of the NHC. The reaction of [RhCl(IPr)2(N2)] 1 (IPr = Af,A -bis-[2,6-(di-tTo-propyl)phenyl]imidazol-2-ylidene) with gave the monomeric complex 3 [1], which was also obtained from the reaction of [RhCl(COE)(IPr)]2 2 with and excess IPr, while the reaction of [RhCl(COE)(lMes)]2 with gave the chloride bridged species 4 (Scheme 2.1) [2],... 

Among these in situ protocols are those using ionic liquids as the solvent, or as both the solvent and the ligand. It was shown that the use of PdCOAc) in imidazolium-based ionic liquids forms in situ NHC-Pd(II) species [42], The use of methylene-bridged bis-imidazolium salt ionic liquids to form chelated complexes has also been reported [43], although better results have been obtained when Bu NBr is used as the solvent [44] and imidazolium salts were added together with PdCl in catalytic amounts [45]. Other related catalytic species such as bis-NHC complexes of silica-hybrid materials have been tested as recyclable catalysts [46,47]. 

A series of complexes of type [Ni(T] -allyl)Cl(NHC)] highlight the important influence of the NHC on the reactivity of the resulting complex towards O. It was shown that 0,-activation is disfavoured when the rotation around the Ni-C, bond is restricted. On the other hand, with complexes displaying free rotation around the Ni-Cj, bond, the complexes react cleanly with O. The overall reaction results in the oxidation of the allyl group and the formation of hydroxy-bridged dimers (Schane 10.4) [14,15]. 

In studies involving the reaction of a cA-[PdCl(Me)(NHC)]j chloro-bridged dimer with CO it was demonstrated that reductive elimination is also extremely facile for NHC-Pd-acyl complexes, yielding 2-acylimidazolium salts and Pd(0) (Scheme 13.4) [22]. The product distribution was shown to depend on the stracture of the complexes from which reductive coupling took place (pathways A and B, Scheme 13.4). 

The broader subject of the interaction of stable carbenes with main-group compounds has recently been reviewed. Accordingly, the following discussion focuses on metallic elements of the s and p blocks. Dimeric NHC-alkali adducts have been characterized for lithium, sodium, and potassium. For imidazolin-2-ylidenes, alkoxy-bridged lithium dimer 20 and a lithium-cyclopentadienyl derivative 21 have been reported. For tetrahydropyrimid-2-ylidenes, amido-bridged dimers 22 have been characterized for lithium, sodium, and potassium. Since one of the synthetic approaches to stable NHCs involves the deprotonation of imidazolium cations with alkali metal bases, the interactions of alkali metal cations with NHCs are considered to be important for understanding the solution behavior of NHCs. 

For rhodium and iridium compounds alkoxo ligands take over the role of the basic anion. Using /z-alkoxo complexes of ( -cod)rhodium(I) and iridium(I)— formed in situ by adding the /r-chloro bridged analogues to a solution of sodium alkoxide in the corresponding alcohol and azolium salts—leads to the desired NHC complexes even at room temperature [Eq. (10)]. Using imidazolium ethoxyl-ates with [(r " -cod)RhCl]2 provides an alternative way to the same complexes. By this method, it is also possible to prepare benzimidazolin-2-ylidene complexes of rhodium(I). Furthermore, an extension to triazolium and tetrazolium salts was shown to be possible. ... 

The use of /r-hydroxo or ju-alkoxo bridged polynuclear complexes of chromium, molybdenum, tungsten, or rhenium in this route leads to the formation of monomeric bis(NHC) complexes, to the elimination of hydrogen, and to the partial oxidation of the metal [Eq.(ll)]. Chelating and nonchelating imidazolium salts as well as benzimidazolium and tetrazolium salts can be used. 

Addition of butyl lithium to a suspension of palladium(II) diiodide and methylene bridged bisimidazolium salts leads to the in situ formation and complexation of the NHC resulting in the cationic [(chelate)2Pd]l2 in low yield. Higher yields... 

NHC ligands have been used to bridge different metals resulting in homo-bimetallic systems. Examples exist for palladium(II), rhodium(I), as well as chromium(O). " Homobimetallic ruthenium(II) systems have been shown to be superior catalysts in cyclopropanation reactions compared to their monometallic... 

A supramolecular, helical structure was obtained from bridging bis(NHC) ligands and Hg(OAc)2 in acetonitrile. ... 

As yet, with regard to both ligands C(NHC)2 and C[C(NMe2)2 2. each is only represented by one transition metal complex. The two complexes are confirmed by X-ray analyses. The carbodicarbene C(NHC)2 was allowed to react with [Rh(p-Cl) (CO)2l2 to afford complex 70 in benzene solution. The carbon is able to split the chlorine bridge in the starting Rh complex and the vacant coordination site is occupied by the ligand, a very common synthetic route [10, 11]. 

More recently, two types of Ru complexes were obtained by the reaction of mesityl-substituted electron-rich olefins with [RuCl2(p-cymene)]2 [27]. Cleavage of the chlorine bridges occurs first to yield the expected (NHC)(p-cymene)Ru(II) complex 9. Under harsher reaction conditions (140 °C in p-xylene) further arene displacement takes place to yield the chelated ( 6-mesityl-NHC - Ru complex 10 (Scheme 6). The olefin 8 was easily obtained by deprotonation of the corresponding dihydroimidazolium salt. 

The most widely used method for the preparation of free NHCs is the deprotonation of an azohum salt with NaH or KOBuf [10,14,37]. In the case of N,N -methylene-bridgcd bisimidazolium salts, the preparation of the free dicarbenes is only possible by the use of potassium hexamethyldisilazide (KHMDS) in toluene [14,41]. Other strong bases deprotonate the methylene bridge breaking the bisazol unit [42],... 

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